1. Field of the Invention
Aspects of the present invention relate to an electrode for a fuel cell and a fuel cell employing the same.
2. Description of the Related Art
Fuel cells employing a polymer electrolyte membrane as an electrolyte can be operated at a relatively low temperature and can be manufactured in small sizes. Fuel cells are expected to be used as power sources for electrical automobiles or household distributed power systems. A perfluorocarbonsulfonate polymer membrane, represented as NAFION (trade name), has been used as a polymer electrolyte membrane in polymer electrolyte membrane fuel cells.
However, such a polymer electrolyte membrane must be humidified for proton conduction. Moreover, in order to increase the efficiency of a fuel cell system, a polymer electrolyte membrane fuel cell must be operated at a high temperature, for example, a temperature of 100° C. or more. However, water is evaporated from an electrolyte membrane at high temperatures, and thus, an electrolyte membrane loses functionality as a solid electrolyte.
In view of the above problem, non-humidified electrolyte membranes that can be operated under non-humidifying conditions, at high temperatures of 100° C. or more have been developed. For example, Japanese Patent Laid-Open Publication No. Hei. 11-503262 discloses phosphoric acid-doped polybenzimidazole or the like, which is used as a material for a non-humidified electrolyte membrane.
In low temperature perfluorocarbonsulfonate polymer electrolyte membrane fuel cells, in order to prevent defective gas diffusion in an electrode (in particular in a cathode), which may be caused by water (product water) generated during electric power production in the electrode, hydrophobic electrodes including polytetrafluoroethylene (PTFE) have been used (e.g., Japanese Patent Laid-Open Publication No. Hei. 05-283082). In high temperature (150-200° C.) phosphoric acid fuel cells, liquid phosphoric acid is used as an electrolyte. However, a large quantity of the liquid phosphoric acid is present at an electrode, thereby hindering gas diffusion. Thus, an electrode that has a layer of water-repellent PTFE that is capable of preventing the blocking of micropores, due to phosphoric acid, has been proposed.
In fuel cells employing a phosphoric acid-impregnated polybenzimidazole (PBI) electrolyte membrane, as a high-temperature, non-humidified electrolyte, in order to promote contact between an electrode and an electrolyte membrane, attempts have been made to impregnate an electrode with liquid phosphoric acid and to load a larger quantity of a metal catalyst. However, the power output characteristics of such fuel cells may not be satisfactory, and thus, there is much room for improvement.
In phosphoric acid-doped solid polymer electrolyte fuel cells, the supply of air to a cathode requires an aging time of about one week, even when using an optimal electrode composition. Although it is possible to improve the performance of the cathode and to reduce the aging time, by replacing the air of the cathode with oxygen, these fuel cells may not be commercially available.